Active cancellation of a wireless coupled transmit signal

ABSTRACT

An active cancellation device receives a model of a first signal from a local wireless transmitter. The first signal causes a coupled signal that interferes in a second signal received by a local wireless receiver from a remote wireless transmitter. The active cancellation device generates a cancellation signal based on the model of the first signal, and provides the cancellation signal to the local wireless receiver. At the local wireless receiver, the cancellation signal combines with the coupled signal and the second signal. The cancellation signal reduces the interference in the second signal caused by the coupled signal.

FIELD OF THE INVENTION

The present invention pertains to the field of wireless communications.More particularly, this invention relates to active cancellation ofwireless transmissions coupled in a wireless receiver.

BACKGROUND

Wireless communications offer increased convenience, versatility, andmobility compared to wireline alternatives. Cellular phones, wirelesscomputer networking, and wireless peripheral components, such as awireless mouse, wireless headphones, or a wireless keyboard, are but afew examples of how wireless communications have permeated daily life.Countless additional wireless technologies and applications are likelyto be developed in the years to come.

Wireless communications use various forms of signals, such as radiofrequency (RF) signals, to transmit data. A transmitter broadcasts asignal from an antenna in a particular frequency band called a passband. As the signal travels, the signal loses power or attenuates. Thefarther the signal travels, the more the signal attenuates.

The signal also encounters various forms of interference along the waythat introduce noise in the signal. The transmitter itself introducesnoise. Signals from other transmitters also introduce noise. A receivertrying to receive the signal is likely to introduce a comparative largeamount of noise. Virtually anything can cause noise, including theground, the sky, the sun, and just about any animate or inanimateobject.

At some distance from the transmitter, the signal will attenuate to thepoint that it becomes lost in noise. When noise overpowers a signal, thesignal and the data it is carrying are often unrecoverable. That is,depending on the distance a signal travels and the amount of noise mixedwith the signal, a receiver may or may not be able to extract thesignal.

A transmitter that is in relatively close proximity to a receiver willintroduce noise in the receiver when the transmitter transmits. Thenoise is called a coupled signal. A coupled signal may introduce so muchnoise that the receiver cannot receive any other signals. Signalcoupling is a major obstacle in wireless communications. Devices thatinclude both a transmitter and a receiver must isolate the transmitterfrom the receiver so that the coupled signal does not overpower thereceiver.

One approach to isolating a transmitter from a receiver is half duplexcommunications. A half duplex device cannot simultaneously send andreceive. A common example is a hand-held, two-way radio. When a userpushes a button to talk into the radio, the user cannot simultaneouslylisten to signals from other radios. That is, the receiver is disabledwhen the transmitter is transmitting. If the receiver were not disabledwhile the transmitter transmits, the transmitter would probably overpower the receiver with noise.

Isolation is particularly troublesome in devices that include more thanone onboard radio. For instance, a portable computer may include morethan one radio to enable more than one simultaneous wireless service. Atransmission from any one radio may over power receivers in multipleradios. One approach to isolating multiple transmitters from multiplereceivers is time division duplex (TDD) communications. In a TDD device,all receivers are disabled when any one transmitter transmits.

A cellular phone, on the other hand, is a full duplex wirelesscommunication device. That is, a cellular phone simultaneously transmitsand receives signals so that a user can talk and listen at the sametime. A cellular phone isolates its transmitter from its receiver byusing two different frequency bands—one band for transmitting and oneband for receiving.

None of these isolation solutions are particularly satisfying. Halfduplex and TDD communications have the obvious disadvantage that a usercannot simultaneously send and receive. This poses a substantialperformance limitation that will become more pronounced as more wirelesscommunications applications and technologies are developed and adopted,and more devices include multiple on-board radios.

Full duplex communications that rely on two isolated frequency bands forsending and receiving data have the obvious disadvantage of using twiceas much frequency bandwidth as half duplex communications. This poses asubstantial performance limitation that will also become more pronouncedas the numbers of competing wireless applications and users continues toincrease, and available bandwidth continues to decrease.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the present invention are illustrated in the accompanyingdrawings. The accompanying drawings, however, do not limit the scope ofthe present invention. Similar references in the drawings indicatesimilar elements.

FIG. 1 illustrates one embodiment of the present invention.

FIG. 2 demonstrates one embodiment of the present invention.

FIG. 3 illustrates one embodiment of a transmitted signal coupled with areceived signal at a wireless receiver.

FIG. 4 illustrates one embodiment of the present invention for narrowband active cancellation.

FIG. 5 illustrates one embodiment of the present invention for wide bandactive cancellation.

FIG. 6 illustrates one embodiment of the present invention for multipleradios.

FIG. 7 illustrates one embodiment of the present invention for activecancellation in general.

FIG. 8 illustrates one embodiment of the present invention for wide bandactive cancellation in the pass band.

FIG. 9 illustrates one embodiment of the present invention for activecancellation in the baseband.

FIG. 10 demonstrates one embodiment of the present invention for activecancellation of several simultaneous transmissions.

FIG. 11 illustrates one embodiment of a hardware system.

FIG. 12 illustrates one embodiment of a machine readable storage medium.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the presentinvention. However, those skilled in the art will understand that thepresent invention may be practiced without these specific details, thatthe present invention is not limited to the depicted embodiments, andthat the present invention may be practiced in a variety of alternateembodiments. In other instances, well known methods, procedures,components, and circuits have not been described in detail.

Parts of the description will be presented using terminology commonlyemployed by those skilled in the art to convey the substance of theirwork to others skilled in the art. Also, parts of the description willbe presented in terms of operations performed through the execution ofprogramming instructions. As well understood by those skilled in theart, these operations often take the form of electrical, magnetic, oroptical signals capable of being stored, transferred, combined, andotherwise manipulated through, for instance, electrical components.

Various operations will be described as multiple discrete stepsperformed in turn in a manner that is helpful in understanding thepresent invention. However, the order of description should not beconstrued as to imply that these operations are necessarily performed inthe order they are presented, or even order dependent. Lastly, repeatedusage of the phrase “in one embodiment” does not necessarily refer tothe same embodiment, although it may.

The present invention reduces the noise effects of a transmitter locatedin close proximity to a receiver. Using various embodiments of thepresent invention, a wireless device can simultaneously transmit andreceive, even within the same frequency band. This is a fundamentalimprovement over the prior art. For instance, where a cellular serviceprovider has enough frequency bandwidth to serve one million prior artcellular phones using two frequency bands per phone, embodiments of thepresent invention may allow two million cellular phones to be served.Similarly, embodiments of the present invention allow multiple halfduplex and/or full duplex radios to be isolated within a single device,even on the same chip.

FIG. 1 illustrates one embodiment of the present invention. Transmitter110 transmits a signal from antenna 140. Receiver 120 receives a signalfrom antenna 140. If transmitter 110 is transmitting at the same timereceiver 120 is receiving, the signal that receiver 120 receivesincludes a coupled signal 150. Active cancellation unit 130 introduces acancellation signal to reduce the coupled signal 150 at receiver 120.

In one embodiment, FIG. 1 represents a full duplex device such as acellular phone. By reducing the coupled signal 150 at receiver 120, afull duplex device may be able to transmit and receive in the samefrequency band.

FIG. 2 demonstrates one embodiment of active cancellation unit 130. At210, the active cancellation unit receives a model of a first signalthat is being transmitted from the local transmitter, transmitter 110.As described below, a signal splitter can be used to split off part ofthe transmitter's output and provide a portion of the transmitted signalto the active cancellation unit. For instance, if the splitter splitsoff ten percent of the transmitter's output, the model of the firstsignal will be a one-tenth replica of the transmitter's output signal.In alternate embodiments, any number of approaches can be used toprovide a replica or copy of the transmitted signal to the activecancellation unit 130.

Meanwhile, the local receiver, receiver 120, is trying to receive asecond signal from a remote transmitter (not shown). The first signalfrom the local transmitter causes interference, or noise, in the secondsignal received from the remote transmitter in the form of coupledsignal 150. The local receiver requires at least a certain minimumsignal to noise ratio to be able to extract the second signal. Theadditional noise from the local transmitter may significantly reduce thesignal to noise ratio at the local receiver. If the second signal isoverpowered by the noise, the receiver may not be able to recover it.

Even if the remote transmitter transmits the second signal at a higheramplitude than the amplitude of the coupled signal, the second signalfrom the remote transmitter will attenuate as it travels. At somedistance from the remote transmitter, the second signal will drop belowthe noise level and become lost. In which case, if the coupled signalcauses an increased level of noise at the local receiver, the coupledsignal will at the very least reduce the range from which the localreceiver can receive the second signal from the remote transmitter.

At 220, the active cancellation unit 130 generates a cancellation signalbased at least in part on the model of the first signal. As discussedbelow, the cancellation signal may be generated in analog and/or digitalcircuitry in a variety of different ways. In various embodiments,generating the cancellation signal involves correlating the model of thefirst signal with the coupled signal at the receiver. That is, thecoupled signal and the model of the first signal take different pathsbefore reaching the active cancellation unit. The different paths mayintroduce different delays, phase shifts, and/or attenuations. Once thesignals are correlated, the cancellation signal can be generated bydelaying, phase shifting, and/or amplitude inverting the correlatedmodel signal to create an inverted replica of the coupled signal as thecoupled signal appears at the local receiver.

In one embodiment, a pilot tone is inserted into the first signal. Inanother embodiment in which the transmitter and receiver operate at thesame center frequency, the pilot tone is the transmitted signal itselfso no additional signal needs to be inserted. In either case, since thepilot tone is part of the first signal, it is also present in the modelof the first signal and the coupled signal. The coupled signal and themodel of the first signal can be correlated by matching the delay,phase, and amplitude of the pilot tone in the model of the first signalto the pilot tone in the second signal.

At 230, the active cancellation unit provides the cancellation signal tothe local wireless receiver to be combined with the received signal. Thecancellation signal destructively interferes with the coupled signal toreduce the noise level in the second signal caused by the first signal.As described below, a signal coupler can be used to combine thecancellation signal with the second signal. In alternate embodiments,any number of approaches can be used to combine the cancellation signalwith the received signal.

If the pilot tone is cancelled from the second signal, then the coupledsignal should also be cancelled. The cancellation can be “active” inthat the signals can be correlated on an on-going basis or at particularintervals so as to adapt to changes in the coupled signal. For instance,as objects move in the vicinity of the transmitter and receiver, or asthe transmitter and receiver are moved or change frequency bands, thepath loss for the coupled signal may change. By repeatedly correlatingthe signals, the active cancellation can adapt to the changes. In oneembodiment, the pilot tone is inserted in the first signal only when thesignals are being correlated. In an alternate embodiment, the pilot toneis continuously present in the transmitted signal.

Coupled signal 150 may be coupled into the received signal in any numberof ways. For instance, if transmitter 110 and receiver 120 are on thesame chip, coupled signal 150 may travel through a ground plan of thechip. Even if transmitter 110 and receiver 120 are physically isolatedand use separate antennas, the coupled signal could pass from oneantenna to the other.

FIG. 3 illustrates one example of a coupled signal from a transmitterthat is overpowering a receiver. In the illustrated embodiment, thetransmitter is transmitting at center frequency 310 and the receiver istrying to receive a different signal 325 from a remote transmitter (notshown) at center frequency 330.

A center frequency is often the frequency of a carrier signal used by atransmitter or receiver. Information can be conveyed over the centerfrequency by, for instance, modulating the carrier frequency and/orphase. A pass band is the minimum frequency bandwidth needed to eitherside of a center frequency for modulation. In alternate embodiments, anynumber of techniques can be used for wireless data transmission, such asamplitude modulation.

In the illustrated embodiment, the pass bands of the transmitter andreceiver do not overlap. The transmitted signal, however, includes bothan intentional signal component 315 generated in the pass band aroundthe center frequency 310 and a broadband noise component 320. Thebroadband noise 320 buries the signal from the remote transmitter.Virtually every transmitter generates noise because virtually everytransmitter includes resistance and virtually every resistor generatesnoise.

Broadband noise 320 is also referred to as white noise. It is broadbandin the sense that it is present in a wide frequency range. It is oftencompletely unpredictable and can completely change from one instant intime to the next. Therefore, any time or phase differential between thebroadband noise 320 and the cancellation signal could render thecancellation signal useless. That is, by the time the propercancellation signal is generated, the white noise at the receiver mayhave completely changed compared to the instant in time from which thecancellation signal was generated.

In various embodiments, to alleviate some timing correlation problems,the broadband noise can be band limited by a pass band filter in thetransmitter to limit the noise to a certain frequency range. By limitingthe broad band noise, the noise is likely to remain constant, orprevented from decorrelating, for at least an instant in time. Ingeneral, if the decorrelation time for the noise is greater than thepropagation delay for the coupled signal, the accuracy of thecancellation signal can be greatly improved. Stated another way, signalcancellation generally improves as the delay path for the coupled signaldecreases. The delay path is often directly related to the distancebetween the transmitter and the receiver. This result provides aninteresting relationship in that both the level of noise coupling andthe accuracy of noise cancellation tend to increase as the distancebetween a transmitter and a receiver decreases.

If the decorrelation time for the noise is not greater than thepropagation delay of the coupled signal, correlating the signals becomesmore difficult. For instance, the received signal may need to be delayedto give the active cancellation unit time to generate the appropriatecancellation signal. In one embodiment, both the cancellation signal andthe received signal are delayed for differing durations in order tocorrelate the signals in time.

Referring back to FIG. 3, the broad band noise is amplified along withthe intentional signal component. The transmitter amplifies the signalto a comparatively high level because the signal attenuates as ittravels. That is, in order for a receiver to receive the signal at somedistant location, the signal must be amplified locally to a level abovethe amplitude needed at the receiver. The higher the amplification, thegreater range at which the signal can be recovered.

Conversely, the signal 325 that the local receiver is trying to receivealso attenuates as it travels. Even if signal 325 was transmitted at acomparatively high amplitude, the signal may be considerably attenuatedby the time it reaches the local receiver. In the illustratedembodiment, in the vicinity of the local transmitter the amplified noiseis higher than the signal 325 from the remote transmitter, effectivelyburying the signal. If the remote transmitter were located closer sothat the signal attenuated less, or if the remote transmittertransmitted at a higher amplitude, the signal to noise ratio at thelocal receive may be high enough to extract the signal. In other words,the signal coupling from the local transmitter at the very least reducesthe range of reception for the local receiver.

FIG. 4 illustrates one embodiment of active cancellation. In theillustrated embodiment, the active cancellation is narrow band, creatinga notch 420 in the coupled signal. For instance, a narrow band activecancellation unit may use configurable delay paths and amplification togenerate an inverted replica of the coupled signal. Segments of delaycan be added or removed so that the difference in delay between thecancellation signal and the coupled signal at the receiver is one halfof a wavelength. Half of a wavelength is 180 degrees out of phase, sothe cancellation signal destructively interferes with the coupledsignal.

The cancellation, however, is narrow because wavelength is a function offrequency. Configurable delay paths can only provide a 180 degree phaseshift at one particular frequency. In other words, accuracy falls off atfrequencies farther away from a center frequency. In which case,performing active cancellation at the center frequency of thetransmitter would only improve the signal to noise ratio for thereceiver if the receiver and transmitter were operating in the samefrequency band. Therefore, as shown in FIG. 4, active cancellation isperformed at the center frequency of the local receiver.

Also illustrated in FIGS. 3 and 4 is the transmission spread spectrumband 340. A spread spectrum transmitter changes the center frequency atwhich it transmits at particular intervals. Spread spectrum transmittingis often used to improve security and/or reception. In the illustratedembodiment, the transmitter's center frequency 310 “hops” from onefrequency to another within the spread spectrum band 340. Since thereceiver's center frequency is also within the spread. Spectrum band340, at some point the transmitter and receiver may operate at the samecenter frequency or the pass bands may overlap. In which case, to theextent that the pass bands of the transmitter and receiver overlap,active cancellation will cancel both the intentional signal component315 and the broadband noise 320.

Narrow band active cancellation tends to be more difficult when thereceiver is a spread spectrum receiver. For instance, each time areceiver's center frequency hops, the configurable delay paths of anarrow band active cancellation unit will need to be reconfigured. Aspread spectrum receiver may frequency hop several thousand times persecond. Reconfiguring delays at that speed may be prohibitively costlyto implement.

FIG. 5 illustrates one embodiment of wide band active cancellation. Inthe illustrated embodiment, the receiver is a spread spectrum receiverand its center frequency hops over the receiver spread spectrum band540. Wide band active cancellation provides a notch 520 in the coupledsignal that is wide enough to encompass the entire receiver spreadspectrum band 540. Rather than centering the notch 520 at the receiver'scenter frequency, the notch can be centered at the center 550 of thespread spectrum band 540. In which case, unlike narrow band activecancellation, wide band active cancellation may not require anyreconfiguring of path delays as the receiver's center frequency hops.

As discussed above, configurable delay paths tend to provide narrow bandactive cancellation because wavelength is a function of frequency soaccuracy falls off at frequencies further away from the centerfrequency. In contrast, certain embodiments of wide band activecancellation rely on a true phase shift that can be applied over a widerfrequency range. Any number of approaches can be used to phase shift andamplify a model signal in order to generate a wide band cancellationsignal. A wide band active cancellation unit is described below withrespect to FIG. 8.

In alternate embodiments, where, for instance, the broad band noise isband limited and/or the transmitter's spread spectrum band and thereceiver's spread spectrum band only partially overlap, wide band activecancellation may be centered around a frequency other than the center ofthe receiver's spread spectrum band. That is, active cancellation mayonly be needed where the transmitter's coupled signal overlaps with thereceiver's spread spectrum. In which case, the notch may be smaller thanthe receiver's spread spectrum and may only encompass the frequencyrange where it is needed.

As a practical matter, active cancellation generally cannot completelycancel a coupled signal. As used herein, “cancellation” refers toimproved isolation. Active cancellation can provide a substantial amountof isolation between a transmitter and receiver. In practice, bothnarrow band and wide band active cancellation have provided over 20 dbof suppression in notches in excess of 40 MHz for narrow band and inexcess of 100 MHz for wide band. These levels of suppression have beenachieved even for transmitters and receivers on the same chip.

FIG. 6 illustrates one embodiment of the present invention for isolatingmultiple radios in the same device or on the same chip. The illustratedembodiment includes both a pass band active cancellation unit 655 and abaseband active cancellation unit 650. In alternate embodiments, activecancellation can be performed using either the pass band or the basebandapproach rather than both.

Transmitter 660A, receiver 665A, digital to analog converter 610A, andanalog to digital converter 630A form a first analog radio. Transmitter660B, receiver 665B, digital to analog converter 610B, and analog todigital converter 630B form a second analog radio. Both radios convertdigital data from a host device (not shown) to analog data fortransmission in analog form and receive analog data from a remote device(not shown) and convert it to digital data for the host device.

The digital to analog converters receive digital samples of a signal. Inone embodiment, each digital sample represents an amplitude measurementof a low frequency analog signal, called a baseband signal, at aparticular instant in time. In which case, the digital to analogconverters generate the analog baseband signal from the samples. Thebaseband signal can carry data in any number of ways. For instance, datacan be represented as amplitude, phase, and/or frequency modulations.The transmitter can convert the baseband signal to a pass band highfrequency carrier signal in any number of ways. For instance, in oneembodiment, the transmitter can multiply, or “mix,” the baseband signalup to the carrier signal frequency. On the receiver side of the radio,the process is reversed.

In one embodiment, both radios are on one or more PCMCIA cards for usein a laptop computer. For instance, both radios could be on the samecard or the radios could be on separate cards in separate card slots.Card slots may be located in an number of positions in a laptopcomputer, including stacked and/or side-by-side. The first radio may bea Bluetooth compliant radio and the second radio may be an IEEE 802.11compliant radio. Both radios can be used for wireless datacommunications and both radios can operate within the same generalfrequency range. One or both radios may have spread spectrum capabilityso that at some point in time the radios are likely to operate in thesame frequency band.

In the illustrated embodiment, each radio is a half duplex device. Thatis, each radio cannot simultaneously send and receive. But, both radioscan be used simultaneously. For instance, both radios can receive at thesame time, both radios can send at the same time, and one radio canreceive while the other radio is sending.

This last situation, in which one radio is sending while the other isreceiving, is essentially the same problem discussed above for the fullduplex device. That is, when one transmitter transmits, it will generatea coupled signal that may over power the neighboring receiver, or atleast reduce the receiver's reception range, unless the coupled signalis cancelled.

Assume for instance that transmitter 660A is transmitting and receiver665B is receiving. Since the radios are half duplex, transmitter 660B isavailable to transmit a pilot tone. The pilot tone could be transmittedat the center frequency of receiver 665B or, for a spread spectrumreceiver with wide band active cancellation, at the center of the spreadspectrum band. Coupler and splitter 640 combines the pilot tone with thesignal from transmitter 660A and provides the combined signal to theantenna 670. Coupler and splitter 640 also splits off a percentage ofthe combined signal and provides it as a model of the combined signal topass band active cancellation unit 655 and, through down converter 633and analog to digital converter 635, to baseband active cancellationunit 650. For instance, the splitter may be an unbalanced splitter whichsends only a small percent of the signal off to the active cancellationunits.

In alternate embodiments, where, for instance, one or more of the radiosare full duplex, a radio's transmitter may not be available to transmita pilot tone. In which case, additional components may be included togenerate the pilot tone. For instance, where the pilot tone is based onthe center frequency of the corresponding receiver, the receiver mayinclude a local oscillator, such as a phase lock loop, to define itscenter frequency. An additional amplifier could be added to the circuitof FIG. 6 to transmit the center frequency from the receiver's localoscillator. For a full duplex radio in which both the transmitter andreceiver share the same frequency band, the known signal from thetransmitter itself could be used as the pilot tone. For wide bandcancellation in which the cancellation notch is not necessarily centeredon the center frequency of the receiver, an additional amplifier couldbe included to transmit at the desired center of the notch.

Of course, a notch may not be entirely symmetric around a pilot tone.For instance, active cancellation may fall off faster at higherfrequencies than at lower frequencies or vice versa. In which case, apilot tone may need to be transmitted at an off-center frequency that isanticipated to generated the desired cancellation notch.

The pilot tone can take any number of forms. In one embodiment, thepilot tone is simply a sinusoid having an amplitude higher than thebroadband noise. In order to conserve power, the pilot tone does nothave to be amplified as much at the signal being transmitted. After all,the pilot tone only needs to travel as far as the neighboring receiver.

In one embodiment of narrow band active cancellation in which the notchis centered at the receiver's center frequency, the pilot tone istransmitted at a particular offset from the receiver's center frequency.This is because a signal component from the receiver itself may coupleinto the received signal. For instance, a local oscillator in a receivermay generate noise at the center frequency of the receiver. In order toprevent the cancellation signal from being correlated to the wrongsignal, a frequency offset can be used to distinguish the pilot tonefrom the receiver's own noise.

Referring again to FIG. 6, at coupler and splitter 645, the receivedsignal is provided to pass band active cancellation unit 655. Pass bandactive cancellation unit 655 may use any number of phase shifters andamplitude adjusters to correlate the pilot tone in the model of thetransmitted signal received from coupler and splitter 640 to the pilottone in the received signal and to generate the cancellation signal. Forinstance, as discussed above, a configurable delay path andamplification would likely provide narrow band active cancellation. Wideband active cancellation, as discussed above, may use a true phase shiftand amplification. An example of wide band active cancellation isdescribed below with respect to FIG. 8.

Pass band active cancellation 655 provides the cancellation signal tocoupler and splitter 645 which combines the signals and provides them tothe appropriate receiver, receiver 665B in this example. Thecancellation signal should reduce the noise level in the receivedsignal. The amount of cancellation depends on the type and accuracy ofthe analog elements used in cancellation unit 655. Depending on thesignal to noise ratio needed to recover the received signal, pass bandcancellation alone may not be enough.

In the illustrated embodiment, the received signal goes through anadditional stage of active cancellation in baseband active cancellationunit 650. Active cancellation in the digital domain operates inessentially the same way as it does in analog. Digital samples of thereceived signal are provided to the cancellation unit 650. Digitalsamples of the transmitted signal are also provided to the cancellationunit 650. The two signals are correlated and a cancellation signal isgenerated, which is an inverted replica of the coupled signal. Thecancellation signal is subtracted from the digital received signal usingadders 620A and 620B.

FIG. 6 illustrates two possible alternate sources for the digitalsamples that model the transmitted signal. One source of digital samplescomes directly from the host device over lines 605. The digital samplesover lines 605 represent the intentional transmitted signal componentwithout the broad band noise. The broad band noise is not introduceduntil later on in the analog transmitters 660A and 660B. In which case,baseband active cancellation unit 650 would only cancel the intentionalcomponent of the transmitted signal to the extent that it overlaps thecancellation notch.

The other source of digital samples includes both the intentional signalcomponent and the broad band noise. The model of the signal coming fromsplitter 640 is a portion of the carrier signal. Down conversion 633converts the model down to the baseband. Analog to digital converter 635converts the baseband signal to digital samples for the baseband activecancellation unit 650. The digital samples include both the intentionalsignal component and the broad band noise. In one embodiment, the twosignals are correlated by matching what is left of the pilot tone in thereceived signal after pass band cancellation to the pilot tone in thetransmitted signal.

FIG. 7 illustrates one embodiment of a generalized active cancellationunit. In some respects, the illustrated embodiment resembles echocancellation in wireline applications. In wireline echo cancellation, afull duplex device sends and receives signals over a wire in the samefrequency band. From the perspective of the wireline device, the channelcharacteristics of the wire can be thought of in terms of a filter. Atransmitted signal is sent out, passes through some filter which causescomplex delay, phase, and amplitude changes, and comes back in the formof an echo to the receiver. Wireline echo cancellation tries to cancelthe echo by reconstructing the channel characteristics of thehypothetical filter.

Echo cancellation, however, is unheard of, and is generally believed tobe impossible, in wireless applications. The filter characteristics of awire are generally fairly static, making the filter easier to construct.A wireless transmission medium is thought to more dynamic, making thefilter harder to construct.

Moreover, wireline echo cancellation is performed at the baseband levelbecause wireline applications do not use high frequency carrier signalsin the same sense that wireless applications do. Wireless echocancellation has to deal with broad band noise at the pass band level.That is, unlike a wireline application, a wireless transmitter receivesa baseband signal and “mixes” it to produce a high frequency carriersignal. In the process of mixing the baseband signal, the transmittergenerates considerable broad band noise. The broad band noise alone maybe enough to overpower a coupled receiver in the pass band portion ofthe receiver before ever reaching the baseband portion.

For instance, a wireless receiver generally includes one or more lownoise amplifiers (LNA) in the pass band portion of the receiver. An LNAamplifier is supposed to amplify a received signal so that, forinstance, the signal is strong enough to be processed by other elementsin the receiver. An LNA is designed to operate within a certain range ofinput power. A coupled signal from a neighboring transmitter may be sostrong that it can exceed the acceptable input range of an LNA,effectively saturating the receiver in the pass band. In other words,baseband wireline echo cancellation techniques would often be useless inwireless applications because the problems begin to occur long beforereaching the baseband.

Furthermore, given that the carrier frequency in wireless applicationsis generally very high relative to the data rate, or the rate of carriermodulations, channel characteristics for wireless active cancellationcan generally be adequately modeled with a delay, phase, and amplitudeadjustment. That is, the modulations of the carrier frequency changerelatively slowly compared to the carrier frequency, so the wirelesschannel characteristics can be modeled with certain simplifications thatare not possible in wireline applications without loosing significantdata.

In the illustrated embodiment, wireless active cancellation is performedusing the three adjusters, delay 710, phase 720, and amplitude 730, togenerate cancellation signal 750. Error calculator 740 receives themodel of the transmitted signal 705 and the received signal 760 anddetermines the error, or the difference, between them. Based on thedifferences, error calculator 740 instructs the adjusters appropriately.The same basic approach can be used in both pass band activecancellation and baseband active cancellation.

If the delay through the coupled path is short compared to thewavelength of the carrier modulations, the delay can be modeled as aphase shift, thereby eliminating the need for a separate delayadjustment. For instance, if the difference in the path delay for thecoupled signal and the model of the transmitted signal is equal to aquarter of a wavelength, the shift between the signals remains withinthe same period or cycle. In which case, the coupled signal can betreated like a signal having different amplitude and phase, but isotherwise exactly the same as the transmitted signal.

FIG. 8 illustrates one embodiment of a wide band active cancellationunit for use at the pass band portion of the receiver. The adjustmentsto phase and amplitude are accomplished in the illustrated activecancellation unit by two bipolar amplifiers 805 and 815, a −90 degreephase shifter 810, and a summer 845. The error calculations areaccomplished by two mixers 825 and 835 and two loop filters 820 and 830.In one embodiment, the received signal 850 is a feedback signal thatincludes both the cancellation signal and the coupled signal. In whichcase, the active cancellation unit operates like a phase lock loop,continuously adjusting cancellation signal 860 to reduce the errorsignal.

Bipolar amplifiers 805 are “bipolar” in the sense that they can amplifyboth positive and negative. Phase shifter 810 provides a quadrature ofthe model 840 of the transmitted signal. A “quadrature” relates to amathematical representation of a signal. A signal can be mapped at anyinstant in time to a quadrant of a complex plane having an axiscorresponding to a real number component of the signal and an axiscorresponding to an imaginary component. Each quadrant corresponds to a90 degree phase of the signal. For instance, the first quarter of asignal's cycle may map to the first quadrant of the complex plane, thesecond quarter may map to the second quadrant, and so on. Therefore, thequadrature signal maps to an adjacent quadrant of the complex plane andhas the same magnitude as the model 840.

The bipolar amplifiers 805 and 815 can be adjusted to positively ornegatively amplify the model 840 and its quadrature, and the results arecombined by summer 845. The combination of these two signals can producea cancellation signal 860 in any of the four quadrants of the complexplane.

The amount of amplification is determined by the error calculationportion of the circuit. The model 840 is mixed with the received signal850 by mixer 825 to produce a proportional error signal. Basically, thisis mixing a signal with itself in that the model 840 and the receivedsignal 850 both include the same transmitted signal structure. Any smallerrors, or differences, between the signals can be attributed todifferences in the transmission paths.

The quadrature signal from phase shifter 810 is mixed with the receivedsignal 850 by mixer 835 to produce a proportional error signal. Smallerrors between these signals can be attributed to differences in thetransmission paths and the −90 degree phase shift.

The in-phase error signal is provided to loop filter 820 and theout-of-phase error signal is provided to loop filter 830. The loopfilters are low pass filters and integrators. Large errors can beattributed to other signal sources, such as the signal the receiver istrying to receive. Therefore, those errors are filtered out. Theintegrators add up all the remaining errors. The combined errors areproportional to the gain desired from the respective bipolar amplifiers805 and 815.

The illustrated embodiment is particularly elegant. For instance, theonly phase shift needed is a −90 degree phase shift which can be doneover a comparatively wide frequency range with a comparatively highdegree of accuracy. This is especially important for wide cancellationnotches. The process can also be done continuously, making thecancellation signal adaptive to changes in the channel characteristics.Having two variables also guarantees convergence to one solution. Thatis, the circuit implements a quadratic in the form of the variable gainof bipolar amplifier 805 and the variable gain of bipolar amplifier 815.Modulation effects in the carrier frequency are automatically removed soit works for virtually any phase or frequency modulation of the carrierfrequency.

Of course, any number of alternate approaches can be used for pass bandactive cancellation. For instance, four phase shifters could be used tomap to the for different quadrants and four variable amplifiers could beused to move the signals around in the various quadrants. As discussedabove, a number of configurable delays and gains could be used. In theillustrated embodiment of FIG. 8, a positive phase shift could be usedinstead of a negative phase shift.

Similarly, baseband active cancellation can be implemented in any numberof ways. FIG. 9 illustrates one embodiment of a baseband activecancellation unit, such as active cancellation unit 650 from FIG. 6. Thebaseband active cancellation unit operates much like the pass bandactive cancellation unit in that it determines an error signal andgenerates a cancellation signal based on the error. The illustratedembodiment is specifically directed to canceling a Bluetooth signal froman 802.11 signal. A similar approach can be used to cancel a coupledsignal from virtually any received signal, including an 802.11 signalfrom a Bluetooth signal.

Error determination in the illustrated embodiment is based on a timetracking loop. Complex negative rotator 920 and positive rotator 921receive a frequency offset 910 for the central frequency of theinterference signal (the Bluetooth signal in this example). Rotator 920also receives digital samples of the received signal. The output ofrotator 920 is provided to early/late correlators bank 940 along with afirst delayed version through delay element 930 and a second delayedversion through delay element 931. Each delayed version is delayed byone-half of the period of the pulse duration of the interference signalso that each correlator correlates the delayed version with a feedbackwaveform 976. Square unit 950 computes the square of the earlycorrelator and the square of the late correlator, and computes bysubtraction the difference to get an error indication. The errorindication is provided to early/late filter 960.

Variable delay unit 970 and shifter 980 generate the cancellationsignal. Variable delay unit 970 receives the model of the transmittedBluetooth signal 915 and the output of early/late filter 960. TheBluetooth signal is delayed to match the 802.11 signal based on theoutput of the early/late filter 960. The delayed Bluetooth signal isphase shifted by shifter 980 and is provided through positive rotator921 to be subtracted from the 802.11 signal by adders 620. Delay 975also receives the delayed Bluetooth signal and provides it as feedbackwaveform 976 to early/late correlators bank 940.

Those skilled in the art will recognize that the present invention cansimilarly be applied to more than two radios, as well as multiple fullduplex radios. For instance, pilot tones could be added to eachtransmitted signal, and multiple cancellation signals could be generatedby correlating to each pilot tone.

FIG. 10 demonstrates one embodiment of the present invention as appliedto multiple radios. A model of a signal transmitted from a localtransmitter is received at 1010. The transmitted signal causes a coupledsignal that interferes with the reception of one or more local wirelessreceivers. The wireless receivers are trying to receive signals fromremote transmitters.

At 1020, if there are additional local transmitters that aretransmitting, the process loops back to receive a model of the signaltransmitted from the additional local transmitters. At 1030, thereceived models are mixed. At 1040, a cancellation signal is generatedfor each local receiver that is experiencing a coupled signal. At 1050,the cancellation signals are provided to the respective receivers.

In an alternate embodiment, if multiple transmitters share an antenna,the transmitted signals can be combined prior to tapping a model of thecombined transmitted signal rather than tapping multiple models of thesignals and mixing the models as suggested in FIG. 10. Any number ofantenna configurations or arrays can be used. For instance, all localtransmitters and receivers can have separate antennas, or they can shareone antenna, or they can share selected antennas in any number ofcombinations.

FIG. 11 illustrates one embodiment of a hardware system intended torepresent a broad category of computer systems such as personalcomputers, workstations, and/or embedded systems. In the illustratedembodiment, the hardware system includes processor 1110 coupled to highspeed bus 1105, which is coupled to input/output (I/O) bus 1115 throughbus bridge 1130. Temporary memory 1120 is coupled to bus 1105. Permanentmemory 1140 is coupled to bus 1115. I/O device(s) 1150 is also coupledto bus 1115. I/O device(s) 1150 may include a display device, akeyboard, one or more external network interfaces, etc.

Certain embodiments may include additional components, may not requireall of the above components, or may combine one or more components. Forinstance, temporary memory 1120 may be on-chip with processor 1110.Alternately, permanent memory 1140 may be eliminated and temporarymemory 1120 may be replaced with an electrically erasable programmableread only memory (EEPROM), wherein software routines are executed inplace from the EEPROM. Some implementations may employ a single bus, towhich all of the components are coupled, or one or more additional busesand bus bridges to which various additional components can be coupled.Those skilled in the art will be familiar with a variety of alternateinternal networks including, for instance, an internal network based ona high speed system bus with a memory controller hub and an I/Ocontroller hub. Additional components may include additional processors,a CD ROM drive, additional memories, and other peripheral componentsknown in the art.

In one embodiment, the present invention, as described above, isimplemented using one or more computers such as the hardware system ofFIG. 11. Where more than one computer is used, the systems can becoupled to communicate over an external network, such as a local areanetwork (LAN), an internet protocol (IP) network, etc. In oneembodiment, the present invention is implemented as software routinesexecuted by one or more execution units within the computer(s). For agiven computer, the software routines can be stored on a storage device,such as permanent memory 1140.

Alternately, as shown in FIG. 12, the software routines can be machineexecutable instructions 1210 stored using any machine readable storagemedium 1220, such as a diskette, CD-ROM, magnetic tape, digital video orversatile disk (DVD), laser disk, ROM, Flash memory, etc. The series ofinstructions need not be stored locally, and could be received from aremote storage device, such as a server on a network, a CD ROM device, afloppy disk, etc., through, for instance, I/O device(s) 1150 of FIG. 11.

From whatever source, the instructions may be copied from the storagedevice into temporary memory 1120 and then accessed and executed byprocessor 1110. In one implementation, these software routines arewritten in the C programming language. It is to be appreciated, however,that these routines may be implemented in any of a wide variety ofprogramming languages.

In alternate embodiments, the present invention is implemented indiscrete hardware or firmware. For example, one or more applicationspecific integrated circuits (ASICs) could be programmed with one ormore of the above described functions of the present invention. Inanother example, one or more functions of the present invention could beimplemented in one or more ASICs on additional circuit boards and thecircuit boards could be inserted into the computer(s) described above.In another example, field programmable gate arrays (FPGAs) or staticprogrammable gate arrays (SPGA) could be used to implement one or morefunctions of the present invention. In yet another example, acombination of hardware and software could be used to implement one ormore functions of the present invention.

Thus, a method and apparatus for active cancellation of a wirelesstransmitted signal coupled in a wireless received signal is described.Whereas many alterations and modifications of the present invention willbe comprehended by a person skilled in the art after having read theforegoing description, it is to be understood that the particularembodiments shown and described by way of illustration are in no wayintended to be considered limiting. Therefore, references to details ofparticular embodiments are not intended to limit the scope of theclaims.

What is claimed is:
 1. A method comprising: receiving a model of a firstsignal from a local wireless transmitter, said first signal to cause acoupled signal that interferes in a second signal received by a localwireless receiver from a remote wireless transmitter; generating acancellation signal based at least in part on the model of the firstsignal and a center frequency identified for the cancellation signal;and providing the cancellation signal to the local wireless receiver tobe combined with the coupled signal and the second signal, saidcancellation signal to reduce interference in the second signal causedby the coupled signal.
 2. The method of claim 1 wherein the localwireless transmitter and the local wireless receiver operate in a samefrequency band.
 3. The method of claim 1 wherein the local wirelesstransmitter and the local wireless receiver comprise one of a fullduplex communications device, components in separate full duplexcommunications devices, components in separate half duplexcommunications devices, and components in a combination of full duplexand half duplex communications devices.
 4. The method of claim 3 whereinthe full duplex communications device comprises a cellular phone.
 5. Themethod of claim 1 wherein the local wireless transmitter and the localwireless receiver comprise a single integrated circuit.
 6. The method ofclaim 1 wherein the local wireless transmitter comprises a Bluetoothtransmitter and the local wireless receiver comprises an IEEE 802.11receiver.
 7. The method of claim 1 wherein the local wirelesstransmitter comprises an IEEE 802.11 transmitter and the local wirelessreceiver comprises a Bluetooth receiver.
 8. The method of claim 1wherein the model of the first signal comprises one output signal from asignal splitter at an output port of the local wireless transmitter. 9.The method of claim 1 wherein an output of the second local wirelesstransmitter is one of coupled to a mixer to mix with an output of thefirst local wireless transmitter, and said second local wirelesstransmitter shares an antenna with the first local wireless transmitter.10. The method of claim 1 wherein generating the cancellation signalcomprises: adjusting and combining one or more gains and phases of themodel of the first signal to generate an inverted replica of the coupledsignal at the local wireless transmitter.
 11. The method of claim 1wherein generating the cancellation signal is performed in at least oneof analog circuitry and digital circuitry.
 12. The method of claim 1further comprising: receiving a plurality of additional models ofadditional signals received from a plurality of additional localwireless transmitters, said additional signals to contribute to thecoupled signal; generating a plurality of additional cancellationsignals based on the model of the first signal and the plurality ofadditional models; and providing selected ones of the plurality ofadditional cancellation signals to corresponding ones of a plurality ofadditional local wireless receivers to be combined with the coupledsignal to reduce interference caused by the coupled signal.
 13. A methodcomprising: receiving a model of a first signal from a local wirelesstransmitter, said first signal to cause a coupled signal that interferesin a second signal received by a local wireless receiver from a remotewireless transmitter; generating a cancellation signal based at least inpart on the model of the first signal; and providing the cancellationsignal to the local wireless receiver to be combined with the coupledsignal and the second signal, said cancellation signal to reduceinterference in the second signal caused by the coupled signal whereinthe coupled signal comprises broad band noise and a signal component ata first center frequency in a first pass band, wherein the second signalis at a second center frequency in a separate pass band, and wherein theinterference caused by the coupled signal is primarily caused by thebroad band noise.
 14. The method of claim 13 wherein the broad bandnoise is band limited such that a decorrelation time of the broad bandnoise is greater than a propagation delay of the coupled signal.
 15. Amethod comprising: receiving a model of a first signal from a localwireless transmitter, said first signal to cause a coupled signal thatinterferes in a second signal received by a local wireless receiver froma remote wireless transmitter; generating a cancellation signal based atleast in part on the model of the first signal; and providing thecancellation signal to the local wireless receiver to be combined withthe coupled signal and the second signal, said cancellation signal toreduce interference in the second signal caused by the coupled signalwherein generating the cancellation signal comprises: identifying acenter frequency for the cancellation signal; and phase shifting andamplifying the model of the first signal around the center frequency forthe cancellation signal to generate an inverted replica of the coupledsignal at the local wireless receiver.
 16. The method of claim 15wherein the center frequency for the cancellation signal comprises acenter frequency of the local wireless receiver.
 17. The method of claim15 wherein the center frequency for the cancellation signal comprises aneffective center of a wide band cancellation notch.
 18. The method ofclaim 15 wherein identifying the center frequency for the cancellationsignal and phase shifting and amplifying the model of the first signalare based on a pilot tone inserted in the first signal.
 19. The methodof claim 18 wherein the pilot tone is intentionally offset apredetermined amount from a center frequency of the local wirelessreceiver.
 20. The method of claim 18 wherein the pilot tone is generatedby an amplifier coupled to a local oscillator that defines a centerfrequency for the local wireless receiver, said amplifier to mix thepilot tone with an output of the local wireless transmitter.
 21. Themethod of claim 18 wherein the local wireless transmitter is a firstlocal wireless transmitter, and wherein the pilot tone is generated by asecond local wireless transmitter associated with the local wirelessreceiver.
 22. The method of claim 18 wherein the pilot tone is insertedat particular intervals.
 23. A method comprising: receiving a model of afirst signal from a local wireless transmitter, said first signal tocause a coupled signal that interferes in a second signal received by alocal wireless receiver from a remote wireless transmitter; generating acancellation signal based at least in part on the model of the firstsignal; and providing the cancellation signal to the local wirelessreceiver to be combined with the coupled signal and the second signal,said cancellation signal to reduce interference in the second signalcaused by the coupled signal wherein generating the cancellation signalcomprises: applying the coupled signal, the second signal, and the modelof the first signal to a digital time tracking loop; adjusting delaysthrough the digital time tracking lop until the model of the firstsignal and the coupled signal are correlated in time; and delaying themodel of the fist signal by one-half of a period so that the coupledsignal and the model of the first signal are 180 degrees out of phase.24. An apparatus comprising: a signal generator; and a controller, saidsignal generator and said controller to receive a model of a firstsignal from a local wireless transmitter, said first signal to cause acoupled signal that interferes in a second signal received by a localwireless receiver from a remote wireless transmitter, said controller tocontrol said signal generator to generate a cancellation signal based atleast in part on the model of the first signal and a center frequencyidentified for the cancellation signal and provide the cancellationsignal to the local wireless receiver to be combined with the coupledsignal and the second signal, said cancellation signal to reduceinterference in the second signal caused by the coupled signal.
 25. Theapparatus of claim 24 wherein the signal generator comprises at leastone of an amplifier, a phase shifter, and a delay adjustor.
 26. Theapparatus of claim 24 wherein the controller comprises: a plurality ofdelay elements; a bank of early/late correlators; a difference unit; andan early/late filter, said plurality of delay elements to providedelayed versions of the model of the first signal to the bank ofearly/late correlators, said bank of early late correlators to correlatethe model and the delayed version to a feedback signal from the signalgenerator and provide a correlated pair of signals to the differenceunit, said difference unit to generate an error indication based on thecorrelated pair of signals, and said early/late filter to filter outlarge errors and provide a filtered error signal to the signalgenerator.
 27. A machine readable medium having stored thereon machinereadable instructions to implement a method comprising: receiving amodel of a first signal from a local wireless transmitter, said firstsignal to cause a coupled signal that interferes in a second signalreceived by a local wireless receiver from a remote wirelesstransmitter; generating a cancellation signal based at least in part onthe model of the first signal and a center frequency identified for thecancellation signal; and providing the cancellation signal to the localwireless receiver to be combined with the coupled signal and the secondsignal, said cancellation signal to reduce interference in the secondsignal caused by the coupled signal.
 28. The machine readable medium ofclaim 27 wherein the local wireless transmitter and the local wirelessreceiver operate in a same frequency band.
 29. The machine readablemedium of claim 27 wherein the local wireless transmitter and the localwireless receiver comprise one of a full duplex communications device,components in separate full duplex communications devices, components inseparate half duplex communications devices, and components in acombination of full duplex and half duplex communications devices. 30.The machine readable medium of claim 27 wherein generating thecancellation signal comprises: applying the coupled signal, the secondsignal, and the model of the first signal to a digital time trackingloop; adjusting delays through the digital time tracking loop until themodel of the first signal and the coupled signal are correlated in time;and delaying the model of the first signal by one-half of a period sothat the coupled signal and the model of the first signal are 180degrees out of phase.
 31. The machine readable medium of claim 27further comprising: receiving a plurality of additional models ofadditional signals received from a plurality of additional localwireless transmitters, said additional signals to contribute to thecoupled signal; generating a plurality of additional cancellationsignals based on the model of the first signal and the plurality ofadditional models; and providing selected ones of the plurality ofadditional cancellation signals to corresponding ones of a plurality ofadditional local wireless receivers to be combined with the coupledsignal to reduce interference caused by the coupled signal.
 32. Amachine readable medium having stored thereon machine readableinstructions to implement a method comprising: receiving a model of afirst signal from a local wireless transmitter, said first signal tocause a coupled signal that interferes in a second signal received by alocal wireless receiver from a remote wireless transmitter; generating acancellation signal based at least in part on the model of the firstsignal; and providing the cancellation signal to the local wirelessreceiver to be combined with the coupled signal and the second signal,said cancellation signal to reduce interference in the second signalcaused by the coupled signal wherein the coupled signal comprises broadband noise and a signal component at a first center frequency in a firstpass band, wherein the second signal is at a second center frequency ina separate pass band, and wherein the interference caused by the coupledsignal is primarily caused by the broad band noise.
 33. A machinereadable medium having stored thereon machine readable instructions toimplement a method comprising: receiving a model of a first signal froma local wireless transmitter, said first signal to cause a coupledsignal that interferes in a second signal received by a local wirelessreceiver from a remote wireless transmitter; generating a cancellationsignal based at least in part on the model of the first signal; andproviding the cancellation signal to the local wireless receiver to becombined with the coupled signal and the second signal, saidcancellation signal to reduce interference in the second signal causedby the coupled signal wherein generating the cancellation signalcomprises: identifying a center frequency for the cancellation signal;and phase shifting and amplifying the model of the first signal aroundthe center frequency for the cancellation signal to generate an invertedreplica of the coupled signal at the local wireless receiver.
 34. Themachine readable medium of claim 33 wherein the center frequency for thecancellation signal comprises a center frequency of the local wirelessreceiver.
 35. The machine readable medium of claim 33 wherein the centerfrequency for the cancellation signal comprises an effective center of awide band cancellation notch.
 36. The machine readable medium of claim33 wherein identifying the center frequency for the cancellation signaland phase shifting and amplifying the model of the first signal arebased on a pilot tone inserted in the first signal.
 37. A machinereadable medium having stored thereon machine readable instructions toimplement a method comprising: receiving a model of a first signal froma local wireless transmitter, said first signal to cause a coupledsignal that interferes in a second signal received by a local wirelessreceiver from a remote wireless transmitter; generating a cancellationsignal based at least in part on the model of the first signal; andproviding the cancellation signal to the local wireless receiver to becombined with the coupled signal and the second signal, saidcancellation signal to reduce interference in the second signal causedby the coupled signal wherein generating the cancellation signalcomprises: adjusting and combining one or more gains and phases of themodel of the first signal to generate an inverted replica of the coupledsignal at the local wireless transmitter.
 38. A method comprising:receiving a model of a first signal from a local wireless transmitter,said first signal to cause a coupled signal that interferes in a secondsignal received by a local wireless receiver from a remote wirelesstransmitter; generating a cancellation signal based at least in part onthe model of the first signal and a pilot tone inserted in the firstsignal; and providing the cancellation signal to the local wirelessreceiver to be combined with the coupled signal and the second signal,said cancellation signal to reduce interference in the second signalcaused by the coupled signal.
 39. The method of claim 38 wherein saidgenerating includes identifying a center frequency for the cancellationsignal based on said pilot tone.
 40. The method of claim 39 wherein saidgenerating includes phase shifting and amplifying the model of the firstsignal around the center frequency for the cancellation signal.
 41. Amethod comprising: receiving a plurality of models of a plurality ofsignals from a plurality of wireless transmitters, respectively, saidplurality of signals to contribute to a coupled signal that interferesin a plurality of incoming signals received by a corresponding pluralityof local wireless receivers from a remote wireless transmitter; mixingthe plurality of models in order to create a combined transmitted signalmodel; generating a plurality of cancellation signals based at least inpart on the combined transmitted signal model; and providing selectedones of the plurality of cancellation signals to corresponding ones ofthe plurality of local wireless receivers to be combined with thecoupled signal to reduce interference caused by the coupled signal.